1. Field of Invention
The invention relates to waste-processing systems for processing manure.
2. Background Prior Art
Many prior art waste-processing systems are designed for low-solids waste, such as municipal waste, that has a solids content of approximately one percent. High-solids wastes such as manure that have a solids content of approximately twelve percent either clog the system or are insufficiently processed. The processing of high-solids waste has typically been performed using a plug flow process that is characterized by a straight-through system.
Prior art waste-processing systems for either high- or low-solids waste use large amounts of purchased energy in the form of electricity or natural gas to generate heat and run pumps to process the wastes because these systems typically exhibit inefficient heating of the waste as it is processed. In addition, prior art waste-processing systems have the added problem of disposing of the products of their processing. It is anticipated that stricter environmental regulations will limit the amount of waste than can be applied to fields as fertilizer because of the phosphates and nitrogen content of the waste. As fields reach their limits, other fields must be found. As the amount of unfertilized land dwindles, either other outlets for waste must be found, or a disposal method that meets the stricter environmental regulations must be developed and used.
The apparatus and method embodying this invention provide a waste-processing system capable of processing high-solids wastes such as manure. Total process flows are controlled in substantially-closed systems to minimize end waste products and maximize energy efficiency. The apparatus and method embodying this invention provide a compact U-shaped digester that allows for recycling of activated sludge to improve the efficiency of the process. Efficiency is also improved through a sludge heating design that creates a current in the digester and efficiently heats the sludge. Resource use is optimized to preclude the need to purchase outside energy, and to minimize the outflow of water that is unusable without further processing. For example, sludge is dried using waste heat from the processes, rather than using heat generated with energy from outside sources. Finally, a composter is provided to further process the sludge through aerobic digestion to create usable fertilizer, thus minimizing the output of unusable waste products.
A digester for processing high-solids waste is provided comprising a mixing chamber, a clarifier, and a generally U-shaped digester. The mixing chamber is located adjacent the clarifier such that activated sludge may be recycled to the mixing chamber. A heater is provided in the digester such that thermal agitation of the waste causes controlled mixing of wastes in the digester.
In another embodiment, gas jets are provided in the digester such that agitation of the waste by the gas jets causes the waste to be mixed.
In another embodiment, a gas turbine is provided that is fueled by biological gases produced in the waste-processing system.
In another embodiment, a composting tank mounted in a water tank is provided to aerobically digest the waste.
In another embodiment, the composter is replaced with a solids dryer using waste heat to dry the sludge.
In another embodiment, a combination of a fluidizing bed dryer and an air/air heat exchanger replaces the solids dryer to dry the solids and recapture heat produced by the turbines that would otherwise be lost in the turbine exhaust. The heated air in the fluidizing bed dryer evaporates water carried in the effluent from the solids press. The latent heat of vaporization carried by the moisture in the air leaving the fluidizing bed dryer is substantially recaptured in the water condenser.
Other features and advantages of the invention are set forth in the following drawings, detailed description and claims.